Ceramic substrates, especially those of glass, are often coated, either locally or completely, with one or more coating compositions. Local application is usually practiced to apply lettering, designs, or other indicia to the ceramic substrates; when used in this manner the coating compositions are generally referred to as "inks" . Application of indicia to glass bottles is a commercially important example.
The coatings applied to bottles must be tough and resistant to marring by abrasion or impact and they should be resistant to degredation by caustic solutions commonly is employed for cleaning bottles.
Many of the bottle coatings now used are "applied ceramic labels" , that is, they are applied as inorganic frits which are then exposed to high temperatures. Applied ceramic labels, however, suffer from one or more disadvantages, such as the presence of heavy metals, low gloss, low color brilliance, the necessity of using high temperatures to melt the frits after application, and often a requirement to subsequently reanneal the labeled bottles.
Organic coatings have been used for bottle coatings, but resistances to abrasion and impact of many of these coatings are typically low, and resistances to degredation by caustic bottle-cleaning solutions have often also been low.
Organic coatings based primarily on epoxy resins, dicyandiamide curing agent, and reactive siloxane, and usually containing various additional components, are known. See, for example, the following United States patents: U.S. Pat. No. 3,468,835, U.S. Pat. No. 3,471,312, U.S. Pat. No. 3,607,349, U.S. Pat. No. 5,346,933, and U.S. Pat. No. 5,411,768.
Unfortunately, due primarily to the presence of the dicyandiamide curing agent, some crosslinking of the coating composition does occur at application temperatures and such crosslinking eventually causes the coating composition to thicken to the point it cannot be applied. Accordingly, a major problem with the prior coatings has been short pot life, where "pot life" is the length of time the coating will remain fluid enough to apply to substrates at application temperatures.